Interferon alpha/beta (IFNalpha/Beta) mediate their antiviral, antiproliferative and immunemodulatory effects through the induction of a set of Interferon Stimulated early response Genes (ISGs). The STAT transcription factors have been identified as an important part of the IFNalpha/beta induced signaling cascade. STAT1 and STAT2 become tyrosine phosphorylated in response to IFNalpha/beta, an event that is mediated by the tyrosine kinases Jak1 and Tyk2. Subsequently, the STAT proteins translocate to the nucleus where they interact with several enhancer elements to induce transcription. The two aims of Part I are focused on characterizing the process of STAT1 nuclear translocation. During the current funding period we have shown that STAT1 is subject to nuclear export, which is regulated by Jak1 independent of tyrosine phosphorylation. In Aim I we will investigate the mechanism by which Jak1 regulates STAT1 nuclear export. As no nuclear import signal has been identified in any of the STAT proteins thus far, we propose in Aim 2 experiments to identify and characterize the nuclear localization sequence of STAT1. Recently, we demonstrated that STAT 1 arginine methylation is required to prevent its interaction with the nuclear STAT-inhibitor PIAS1 and thus allow for DNA binding and transcription. We also identified additional modification of STAT1 and PIAS1 by ubiquitin-like molecules (SUMO) as a consequence of cytokine stimulation. Part II of the proposal contains two aims that investigate the role of STAT1 modification by ubiquitin and the ubiquitin-like molecule, SUMO-1. In Aim 3 we will identify the sites of STAT1 and PIAS1 ubiquitination and SUMOylation, and determine the biological consequences of mutations in these sites. In Aim 4 we will characterize the E3 ligase activity mediating STAT1 and PIAS1 ubiquitination/SUMOylation. In studying the immunemodulatory function of STAT1, we discovered that IFNbeta acts as a potent survival factor for primary, resting single positive T lymphocytes, but only in the absence of STAT1. The two aims of Part III focus on the elucidation of the anti-apoptotic effects of IFNalpha/beta in STAT 1 deficient primary T lymphocytes. Results from these proposed studies will not only improve our understanding of the mechanism of STAT activation, but will also shed light on the processes involved in the subcellular distribution of these signaling molecules. The investigation of the STAT1 independent mechanism by which IFNbeta prevents T cell apoptosis will elucidate additional IFNalpha/beta induced signaling pathways.